1. Field of the Invention
This invention relates to circuit protection devices comprising elements composed of PTC material.
2. Summary of the Prior Art
It is well known that the resistivity of many conductive materials changes with temperature, and those which show a sharp increase in resistivity over a particular temperature range are frequently referred to as Positive Temperature Coefficient (PTC) materials. Examples of PTC materials are certain doped ceramics such as barium titanate, and certain conductive polymers, i.e. polymers having a particulate conductive filler dispersed therein. The ceramic PTC materials currently available have very stable electrical properties. On the other hand at relatively high voltage gradients, e.g. above 2 volts/mil, their resistivity tends to fall undesirably rapidly after reaching a peak; even more importantly, their minimum resistivity at 25.degree. C. is relatively high, generally above 40 ohm. cm, so that they cannot be used in situations which require a combination of low resistance and small size. In addition they are brittle and difficult to shape. Conductive polymer PTC materials are generally comparatively flexible and easily shaped, and although until recently the range of physical and electrical properties available was limited, recent research has shown how conductive polymers having a much wider range of properties (and in particular low resistivities at lower temperatures combined with high resistivities at higher temperatures) can be prepared, and how improved devices containing conductive polymers can be made. Reference may be made, for example, to U.S. Pat. No. 3,858,144 and to copending and commonly assigned applications Ser. No. 601,638 (Horsma et al.), U.S. Pat. No. 4,177,376, now abandoned Ser. No. 750,149 (Kamath et al.) Ser. No. 751,095 (Toy et al.), now abandoned, Ser. No. 798,154 (Horsma), now abandoned and Ser. No. 873,676 (Horsma) and to application Ser. No. 965,343 of van Konyenburg et al entitled Low Resistivity PTC Compositions, filed Dec. 1, 1978, and the application of Middleman et al entitled PTC Devices Comprising Oxygen Barrier Layers, application Ser. No. 965,345, filed Dec. 1, 1978 now abandoned. The disclosures of this patent and these applications are incorporated by reference herein. PTC ceramic materials have been used in heaters and in circuit protection devices (see for example the article entitled "The PTC Resistor" by R. F. Blaha in Proceedings of the Electronic Components Conference, 1971). However, for circuits which carry a relatively high steady current, for example 1/2 amp or more, circuit protection devices based on PTC ceramic materials are (because of the high resistivity of such materials) unacceptably large in size. Conductive polymer PTC materials have been widely used in heaters. In addition, their use in circuit protection devices has been proposed in the literature, for example in U.S. Pat. Nos. 2,978,665 (Vernet et al.) and 3,243,753 (Kohler). However, in general there is no disclosure in the literature of any specific circuit protection device containing a conductive polymer PTC element which is even alleged to give satisfactory results, and our investigations have shown that where the literature indicates that specific circuit protection devices of this kind have in fact been made, the directions given are inadequate and misleading.